C-nitrosoaniline compounds and their blends as polymerization inhibitors

ABSTRACT

Disclosed herein is a method for inhibiting the premature polymerization of ethylenically unsaturated monomers comprising adding to said monomers an effective amount of at least one inhibitor selected from the group consisting of C-nitrosoaniline and quinone imine oxime compounds. Also disclosed is a composition of matter comprising: 
     A) an ethylenically unsaturated monomer and 
     B) an effective inhibiting amount, sufficient to prevent premature polymerization during distillation or purification of said ethylenically unsaturated monomer, of at least one inhibitor selected from the group consisting of C-nitrosoaniline and quinone imine oxime compounds used together with an effective amount of oxygen or air to enhance the inhibiting activity of said inhibitor.

This application claims the benefit of U.S. Provisional Application No.60/240,084, filed Oct. 16, 2000, entitled C-NITROSOANILINE COMPOUNDS ANDTHEIR BLENDS AS POLYMERIZATION INHIBITORS and to U.S. ProvisionalApplication No. 60/240,082, filed Oct. 16, 2000, entitledQUINONEDIIMINEOXIME COMPOUNDS AND THEIR BLENDS AS POLYMERIZATIONINHIBITORS.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to the use of at least oneC-nitrosoaniline compound and/or quinone imine oxime, alone or incombination with at least one stable nitroxide free radical compound,and/or at least one nitroaromatic compound, and/or at least one quinonealkide compound, preferably a quinone methide, and/or at least onequinone compound, and/or at least one hydroquinone compound, and/or atleast one hydroxylamine compound, and/or at least one phenylenediaminecompound, and/or air or oxygen to inhibit the polymerization ofethylenically unsaturated monomers.

2. Description of Related Art

Many ethylenically unsaturated monomers undesirably polymerize atvarious stages of their manufacture, processing, handling, storage, anduse. A particularly troublesome problem is equipment fouling caused bypolymerization in the purification stages of the production processes ofsuch monomers. Polymerization, such as thermal polymerization, duringtheir purification results in the loss of the monomer and a loss inproduction efficiency owing to the deposition of polymer in or on theequipment being used in the purification, the deposits of which must beremoved from time to lime. Additionally, the formation of solublepolymer leads to loss of monomer, i.e., a lower yield, and an increasein the viscosity of any tars that may be produced. The processing of thetars then requires higher temperature and work (energy cost) to removeresidual monomer.

A wide variety of compounds has been proposed and used for inhibitinguncontrolled and undesired polymerization of ethylenically unsaturatedmonomers. There remains a need, however, for an inhibitor that not onlyprovides highly effective inhibition of polymerization during normaloperation of a continuous manufacturing or purification process, butalso provides satisfactory protection in the event of a loss ofcontinuous inhibitor feed. While many inhibitors are known to providesufficient protection in one of these scenarios, they have not beenfully satisfactory under both normal and upset operating conditions.Accordingly, a substantial need continues in the art for improvedcompositions for inhibiting the polymerization of such monomers duringtheir production and during the distillation process for purifying orseparating them from impurities, as well as during transport andstorage.

Aromatic nitroso and di-nitroso compounds are known to be useful aschemical agents capable of promoting the formation of filler-elastomerlinkages. The aromatic nitroso compounds may be aromatic amines,including polyamines or phenolic compounds. They are also known to beuseful intermediates in the production of other chemicals, such asp-aminodiphenylamine.

U.S. Pat. Nos. 3,988,212 and 4,341,600 disclose the use ofN-nitrosodiphenylamine combined with dinitro-cresol derivatives forinhibiting the polymerization of vinyl aromatic compounds under vacuumdistillation conditions.

U.S. Pat. No. 4,362,893 discloses that C-nitrosodiarylamines may beprepared in a single stage from diarylamines by adding alcoholicsolutions of a hydrogen halide below the surface of a stirred mixture ofthe diphenylamine in an organic liquid containing water.

U.S. Pat. No. 4,479,008 discloses a process for preparingp-nitrosodiphenylamine hydrochloride from N-nitrosodiphenylamine andhydrogen chloride. The product is prepared in the absence of an aromaticsolvent and using a solvent consisting essentially of aliphatic C₅-C₁₀alcohol.

U.S. Pat. No. 4,518,803 discloses a process for the preparation ofp-nitrosodiphenylamine hydrochloride comprising reacting diphenylamine,C₅-C₁₀ alkyl nitrite and anhydrous HCl in the presence of a C₅-C₁₀aliphatic alcohol and essentially in the absence of an aromatic solvent.

U.S. Pat. No. 5,001,171 discloses that uncured modified rubbercompositions comprising mixtures of elastomers and a reinforcing fillercan be prepared by a process which comprises, inter alia, the use of atleast one chemical agent capable of promoting the formation offiller-elastomer linkages. Examples of such useful chemical agentsinclude aromatic furazan oxides, heterocyclic di-N-oxides,1-hydroxy-benzimidazole-3-oxide compounds,1,3-dihydroxy-benzimidazolinone compounds, and aromatic nitrosocompounds.

U.S. Pat. No. 5,623,088 discloses a method of producing4-aminodiphenylamine (4-ADPA) wherein aniline or substituted anilinederivatives and nitrobenzene are reacted under suitable conditions toproduce 4-nitrodiphenylamine or substituted derivatives thereof and/ortheir salts, either or both of which are subsequently reduced to produce4-ADPA or substituted derivatives thereof. The 4-ADPA or substitutedderivatives thereof can be reductively alkylated to producep-phenylenediamine products or substituted derivatives thereof which areuseful as antiozonants. A second embodiment of the invention is thetetrasubstituted ammonium salts or alkyl substituted diammonium salts of4-nitrodiphenylamine, 4-nitrosodiphenylamine and the substitutedderivatives thereof wherein each substituent of the tetrasubstitutedammonium ion is independently selected from the group consisting ofalkyl, aryl and arylalkyl groups and each alkyl substituent of the alkylsubstituted diammonium salt is independently selected.

U.S. Pat. No. 5,648,543 discloses a process for producing a4-nitrosodiphenylamine of the formula

wherein R₁ and R₂ independently represent hydrogen atom, methyl group,ethyl group, cyclohexyl group, methoxy group, ethoxy group or chlorineor bromine atom, or a salt thereof, comprising treating a diphenylaminerepresented by the formula

wherein R₁ and R₂ are as defined above with (i) a mixture of nitrogenoxides, (ii) a hydrogen halide and (iii) an aliphatic alcohol, whereinthe atomic ratio of oxygen to nitrogen of the mixture of nitrogen oxidesis more than 1.0 and less than 2.0.

U.S. Pat. No. 5,739,403 discloses the production of4-aminodiphenylamines by reacting optionally substituted aniline withoptionally substituted nitrobenzene in the presence of water and/oralcohols and organic and/or inorganic bases and then catalyticallyhydrogenating the resultant nitro- and/or nitrosodiphenylamine in thepresence of water. The catalytic hydrogenation of the reaction mixtureis performed in the presence of 25 to 80 weight percent of water,relative to the weight of the reaction mixture from the condensationreaction, the hydrogenation catalyst is removed from the hydrogenationmixture once absorption of hydrogen has ceased, 10 to 100 vol. percentof aromatic solvent, relative to the total volume of the hydrogenationmixture, is optionally added to the hydrogenation mixture, the resultantorganic phase is separated in order to isolate the 4-aminodiphenylamineand the aqueous phase is returned to the initial reaction mixture.

Quinone methides, quinones, hydroquinones, bydroxylamines, and nitroxylcompounds are known polymerization inhibitors.

Quinone methides act mainly as retarders, giving a significant amount ofpolymer during normal inhibition usage but providing protection in theevent of a plant upset during monomer purification by slowing the rateof polymer formation under static conditions. Because of the poor normalinhibition performance, quinone methides must be used in fairly highdosages, making them not very economical to use.

U.S. Pat. Nos. 4,003,800 and 4,040,911 disclose the use of quinonealkides in a styrene purification process.

The following patents, assigned to Ciba-Geigy Corporation or CibaSpecialty Chemicals Corporation, relate to quinone methides and usesthereof.

U.S. Pat. Nos. 5,583,247, 5,670,692, and 5,750,765 disclose theprotection of ethylenically unsaturated monomers from prematurepolymerization during manufacture and storage by the incorporationtherein of an effective stabilizing amount of a quinone methide compoundhaving an electron withdrawing substituent at the 7-methylene group.

U.S. Pat. No. 5,616,774 discloses the protection of ethylenicallyunsaturated monomers from premature polymerization during manufactureand storage by the incorporation therein of an effective stabilizingamount of a 7-aryl quinone methide compound wherein the 7-arylsubstituent is 2-, 3-, or 4-pyridyl, 2- or 3-thienyl, 2- or 3-pyrryl, 2-or 3-furyl, aryl of six to 10 carbon atoms, or said aryl substituted byone to three alkyl of one to eight carbon atoms, alkoxy of one to eightcarbon atoms, alkylthio of one to eight carbon atoms, alkylamino of oneto eight carbon atoms, dialkylamino of two to eight carbon atoms,alkoxycarbonyl of two to eight carbon atoms, hydroxy, nitro, amino,cyano, carboxy, aminocarbonyl, chloro, or mixtures of said substituents.The combination of these quinone methides with at least one stablenitroxyl compound is also disclosed.

U.S. Pat. No. 5,912,106 discloses a method of improving the quality andresolution of photoimages by incorporating into the photocurable resincomposition to be used a selected amount of a polymerization inhibitorso that photopolymerization of the photocurable resin is inhibited inthose areas not directly impinged by light. Inhibitors that can be usedare selected from the group consisting of N-oxyl or nitroxide compounds,quinone methides, nitroso compounds, phenothiazine and selected phenols.

Hindered nitroxyl compounds are known to be very active inhibitors offree radical polymerizations of unsaturated monomers such as styrene,acrylic acid, methacrylic acid, and the like.

U.S. Pat. No. 3,163,677 discloses N,N,O-trisubstituted hydroxylaminesand N,N-disubstituted nitroxides of the formulas:

wherein R₁, R₂, and R₃ are each an alkyl radical having 1 to 15 carbonatoms. (As used herein, the designation N—O* denotes a stable freeradical wherein the asterisk is an unpaired electron.) TheN,N,O-trisubstituted hydroxylamines can be used to make theN,N-disubstituted nitroxides, which are stable free radicals and aresaid to be useful as polymerization inhibitors.

U.S. Pat. No. 3,267,132 discloses that the polymerization of unsaturatednitriles can be greatly inhibited by incorporating therein a minoramount of a nitroso compound selected from the group consisting ofp-nitrosodiarylamines and N-nitrosoarylamines.

U.S. Pat. No. 3,334,103 discloses that nitroxides can be prepared fromthe corresponding heterocyclic amine wherein the nitrogen atom of thenitroxide group is attached to other than a tertiary carbon of analiphatic group (i.e., the nitrogen atom forms a part of a heterocyclicnucleus). These nitroxides are said to have useful properties similar tothose described for the N,N-disubstituted nitroxides of U.S. Pat. No.3,163,677.

U.S. Pat. No. 3,372,182 discloses that a great variety ofN,N-disubstituted, stable, free radical nitroxides not otherwise readilyavailable can be prepared by a simple and convenient process thatcomprises pyrolyzing in an inert reaction medium virtually anyhydroxylamine that is susceptible to cleavage of the O—C bond, e.g.,tri-t-butylhydroxylamine.

U.S. Pat. No. 3,422,144 discloses stable, free radical nitroxides of theformula:

wherein R is selected from the group consisting of tertiary alkyl, aryl,alkaryl, haloaryl, carboxyaryl, alkoxyaryl, alkylthioaryl, pyridyl, anddialkylaminoaryl, and R′ is tertiary alkyl. These nitroxides are said tobe useful as traps for reactive free radicals both in the counting offree radicals and for inhibiting oxidation and free radicalpolymerization.

U.S. Pat. No. 3,494,930 discloses free radicals of the nitroxide typefor use as initiators of free radical reactions, collectors of freeradicals, polymerization inhibitors or antioxidants. They areconstituted by nitrogenous bicyclic compounds in which one of thebridges comprises solely the nitroxide radical group and, in particular,by aza-9-bicyclo (3,3,1) nonanone-3-oxyl-9, and by aza-9-bicyclo (3,3,1)nonane oxyl-9.

U.S. Pat. No. 3,966,711 teaches that 2,2,7,7-tetraalkyl- and2,7-dispiroalkylene-5-oxo-1,4-diazacycloheptanes substituted in the4-position by mono- or tetravalent radicals are powerfullight-stabilizers for organic polymers. They are said to possess highercompatibility than their 4-unsubstituted homologues, from which they canbe synthesized by reactions known for N-alkylation. Preferredsubstituents in the 4-position are alkyl, alkylene, alkenyl, aralkyl,and esteralkyl groups. The 1-nitroxyls derived from the imidazolidinesby oxidation with hydrogen peroxide or percarboxylic acids are also saidto be good light stabilizers.

U.S. Pat. No. 4,182,658 discloses a method for preventing thepolymerization of a readily polymerizable vinyl aromatic compound duringdistillation at elevated temperatures within a distillation apparatusthat is subject to an emergency condition, such as a power outage. Thismethod comprises force-feeding a supplemental polymerization inhibitorhaving a high solubility in the vinyl aromatic compound and a longduration of efficiency into each of the distillation vessels of aconventional distillation apparatus in an amount sufficient to preventpolymerization therein.

U.S. Pat. No. 4,665,185 discloses a process for the efficientpreparation of nitroxyls of sterically hindered amines by the oxidationof the amine using a hydroperoxide in the presence of a small amount ofa metal ion catalyst, at moderate temperature for a short period oftime, to give the nitroxyl in high yield and purity.

U.S. Pat. No. 4,774,374 discloses a vinyl aromatic compositionstabilized against polymerization comprising (a) a vinyl aromaticcompound and (b) an effective amount of a stabilizer system in which theactive ingredient consists essentially of an oxygenated species formedby the reaction of oxygen and an N-aryl-N′-alkyl-p-phenylenediamine.Also disclosed is a process for inhibiting the polymerization of vinylaromatic compounds employing such an oxygenated species.

U.S. Pat. No. 5,254,760 teaches that the polymerization of a vinylaromatic compound, such as styrene, is very effectively inhibited duringdistillation or purification by the presence of at least one stablenitroxyl compound together with at least one aromatic nitro compound.

U.S. Pat. No. 5,504,243 discloses a method for inhibiting polymerizable(meth)acrylic acid and esters thereof from polymerizing during theirproduction, transportation and storage by using as the inhibitor N-oxylcompound and more than one compound selected from the group consistingof manganese salt compound, copper salt compound,2,2,6,6,-tetramethylpiperidine compound and nitroso compound. The N-oxylcompound is one or more kinds selected from2,2,6,6,-tetramethylpiperidinooxyl,4-hydroxy-2,2,6,6,-tetramethylpiperidinooxyl and4,4′,4″-tris-(2,2,6,6,-tetramethylpiperidinooxyl)phosphite. The combineduse of the inhibitors is said to provide a superior inhibiting effect touse alone.

U.S. Pat. Nos. 5,545,782 and 5,545,786 disclose that nitroxyl inhibitorsin combination with some oxygen reduce the premature polymerization ofvinyl aromatic monomers during the manufacturing processes for suchmonomers. Even small quantities of air used in combination with thenitroxyl inhibitors are said to result in vastly prolonged inhibitiontimes for the monomers.

U.S. Pat. No. 5,711,767 discloses that the use of nitroxide compoundsalone or in combination with aromatic amines, such as substitutedphenylenediamines, or phenolic antioxidants provides an effective way toprevent oxidative degradation and gum formation in gasolines.

U.S. Pat. No. 5,910,232 teaches that inhibition performance in styreneprocessing is improved through the addition of a stable nitroxide freeradical compound to the styrene feed and to the reflux of at least onecolumn. A nontoxic retarder, such as phenylenediamine, may alsooptionally be added to the styrene feed and to the reflux.

U.K. Patent Number 1,127,127 discloses that acrylic acid can bestabilized against polymerization by the addition thereto of a nitroxidehaving the essential skeletal structure:

wherein R₁, R₂, R₃, and R₄ are alkyl groups and no hydrogen is bound tothe remaining valencies on the carbon atoms bound to the nitrogen. Thetwo remaining valencies that are not satisfied by R₁ to R₄ or nitrogencan also form part of a ring (e.g., 2,2,6,6tetramethyl-4-hydroxy-piperidine-1-oxyl).

European Patent Application 0 178 168 A2 discloses a method forinhibiting the polymerization of an α,β-ethylenically unsaturatedmonocarboxylic acid during its recovery by distillation by using anitroxide free radical.

European Patent Application 0 765 856 A1 discloses a stabilized acrylicacid composition in which the polymerization of the acrylic acid isinhibited during the distillation process for purifying or separatingthe acrylic acid as well as during transport and storage. Thecompositions comprise three components: (a) acrylic acid, (b) a stablenitroxyl radical, and (c) a dihetero-substituted benzene compound havingat least one transferable hydrogen (e.g., a quinone derivative such asthe monomethyl ether of hydroquinone (MEHQ)). During the distillationprocess, transport, and storage, components (b) and (c) are present in apolymerization-inhibiting amount. During the distillation process,oxygen (d) is preferably added with components (b) and (c).

WO 97/46504 concerns substance mixtures containing: (A) monomerscontaining vinyl groups; and (B) an active amount of a mixture whichinhibits premature polymerization of the monomers containing vinylgroups during their purification or distillation and contains: (i)between 0.05 and 4.5 weight percent, relative to the total mixture (B),of at least one N-oxyl compound of a secondary amine which has nohydrogen atom at the α-C atoms; and (ii) between 99.95 and 95.5 weightpercent relative to the total mixture (B), of at least one nitrocompound. The publication also discloses a process for inhibiting thepremature polymerization of monomers, and the use of mixture (B) forinhibiting the premature polymerization of monomers.

WO 98/14416 discloses that the polymerization of vinyl aromatic monomerssuch as styrene is inhibited by the addition of a composition of astable hindered nitroxyl radical and an oxime compound.

WO 98/25872 concerns substance mixtures containing: (A) compoundscontaining vinyl groups; (B) an active amount of a mixture whichinhibits premature polymerization of the compounds containing vinylgroups and contains: (i) at least one N-oxyl compound of a secondaryamine which does not carry any hydrogen atoms on the a-carbon atoms; and(ii) at least one iron compound; (C) optionally nitro compounds; and (D)optionally co-stabilizers. The publication also discloses a process forinhibiting the premature polymerization of compounds (A) containingvinyl groups, and the use of (B) optionally mixed with nitro compounds(C) and/or co-stabilizers (D) for inhibiting the prematurepolymerization of radically polymerizable compounds and stabilizingorganic materials against the harmful effect of radicals.

WO 99/20584 discloses that polymerization can be inhibited during theanaerobic production of styrene through the addition of a combination ofa stable nitroxide free radical compound and a nontoxic phenylenediaminecompound.

CS-260755 B1 is directed to the preparation of4-substituted-2,2,6,6-tetramethylpiperidine nitroxyls as olefinstabilizers.

Hung. 150,550 discloses that free radical polymerization was inhibitedwith organic nitroso compounds, e.g., p-H₂C₆H₄NO (I),α-nitroso-β-naphthol, or β-nitroso-α-naphthol. For example, addition of0.3 grams of (1) to one liter of styrene is said to have resulted in thestability of the latter for months. Also, (I) could be removed withazodiisobutyronitrile.

SU-334845 A1 is directed to the inhibition of the radical polymerizationof oligoester acrylates using iminoxyl radical inhibitors of a givenformula.

SU-478838 is directed to the inhibition of the radical polymerization ofoligoester acrylates and the prevention of oligomeric peroxides using abinary polymerization inhibitor comprising quinone.

FR 2,761,060 relates to the prevention of premature polymerization ofstyrene during its production by dehydrogenation of ethylbenzene byinjecting into the process effluent a radical inhibitor based on anoxyl-tetramethylpiperidine derivative.

U.S. Pat. No. 4,086,147 discloses a process using 2-nitro-p-cresol as apolymerization inhibitor.

U.S. Pat. Nos. 4,105,506 and 4,252,615 disclose a process using2,6-dinitro-p-cresol as a polymerization inhibitor.

U.S. Pat. Nos. 4,132,602 and 4,132,603 disclose the use of a halogenatedaromatic nitro compound as a polymerization inhibitor for use during thedistillation of vinyl aromatic compounds.

U.S. Pat. No. 4,466,904 discloses the use of phenothiazine,4-tert-butylcatechol and 2,6dinitro-p-cresol as a polymerizationinhibitor system in the presence of oxygen during heating of vinylaromatic compounds.

U.S. Pat. No. 4,468,343 discloses a composition and a process forutilizing 2,6-dinitro-p-cresol and either a phenylenediamine or4-tert-butylcatechol in the presence of oxygen to prevent thepolymerization of vinyl aromatic compounds during heating.

European patent application 240,297 A1 teaches the use of a substitutedhydroxylamine and a dinitrophenol to inhibit the polymerization of avinyl aromatic compound at elevated temperatures in a distillationprocess.

Georgieff, K. K., J. Appl. Polymer Sci. 9(6):2009-18 (1965) measured theinhibitory effect of the following compounds on the bulk polymerizationof methyl methacrylate: hydroquinone, p-tert-butylcatechol,p-methoxyphenol, 2,4-dichloro-6-nitrophenol, n-propyl gallate,di-tert-butyl-p-cresol, 2,2′-methylenebis(4-methyl-6-tert-butylphenol),1-amino-7-naphthol, p-benzoquinone, 2,6-dichloro-benzoquinone,2-amino-1,4-naphthoquinone, three aminoanthraquinones, diphenylamine,p-nitrosodimethylaniline, α- and β-naphthylamine, phenothiazine,N-nitroso-dimethylamine, hexamethylphosphoramide, n-dodecyl mercaptan,benzenethiol, 2,2-diphenyl-1-picrylhydrazyl, phenyl hydrazine,divinylacetylene, and various antimony and copper salts. Polymerizationwas carried out in a test tube in a bath at 101.2° C., benzoyl peroxidebeing used as initiator. Generally, phenols and naphthols were thestrongest inhibitors, followed by quinones, aromatic amines,2,2-diphenyl-1-picrylhydrayl, antimony pentachloride, phenyl hydrazine,divinylacetylene, and the thiols.

Additionally, see also:

JP 62187710 (1987) which includes a C-nitrosoaniline derivative as apolymerization inhibitor of acrylamides at 100° C.;

JP 58014424 (1983) which includes a C-nitrosoaniline derivative as apolymerization inhibitor of aqueous solutions of acrylate esters;

JP 45017652 (1970) which includes a C-nitrosoaniline derivative as apolymerization inhibitor of aqueous solution of acrolein ormethacrolein;

JP 49125315 (1974) which includes C-nitrosodiphenylamine as apolymerization inhibitor of methacrylate and acrylate esters;

JP 53-33578 (1978) which includes C-nitrosodiphenylamine as apolymerization inhibitor; and

Boguslavskaya, L. S., Khim. Prom-st. 43(10):749-52 (1967).

Several articles have described the use of C-nitrosoaniline derivativesas inhibitors of AIBN- or benzoylperoxy-initiated polymerizations ofstyrene, methylacrylate, methyl methacrylate, acetonitrile, and theircopolymers, e.g.,

Tudos, F. et. al., Eur. Polym. J, 18(4):295-9(1982);

Ibid., 19(7):593-5 (1983);

Ibid., 8(11):1281-9 (1972);

Ibid., 30(12):1457-9 (1994);

Ibid., 19(3):225-9 (1983);

Ibid., 19(2):153-7 (1983);

Ibid, 18(6):487-91 (1982);

Tudos, F., Proc. IUPAC Macromol. Symp., 28^(th), 90 (1982);

Tudos, F. et. al., Kinet. Mech. Polyreactions, Int. Symp. Macromol.Chem., Prepr., 5(25):109-113 (1969);

Yoneda, A. et al., Kobunshi Kagaku, 27(300):269-75 (1970); and

Zaitsev, Y. S. et al., Dopov, Akad Nauk Ukr. RSR, Ser. B: Geol., KhimBiol. Nauki, (11):988-91 (1977).

The foregoing are incorporated herein by reference in their entirety.

SUMMARY OF THE INVENTION

N-nitroso compounds and C-nitrosophenols are known as polymerizationinhibitors, especially under the conditions for monomer production andprocessing. It has now been discovered that C-nitrosoaniline compoundsare very effective polymerization inhibitors as well. Additionally,these compounds can be used in conjunction with nitroxyls,nitroaromatics, quinone methides, quinones, hydroxylanines,hydroquinones, phenylenediamines, air, or combinations thereof(hereinafter referred to as “additional compounds”) to provide anenhanced inhibitor product for use during processing of unsaturatedmonomers, especially styrene and acrylates.

More particularly, the present invention is directed to a method forinhibiting the premature polymerization of ethylenically unsaturatedmonomers comprising adding to said monomers an effective amount of atleast one inhibitor selected from the group consisting ofC-nitrosoaniline and quinone imine oxime compounds.

In another aspect, the present invention relates to a method fordistilling a feed comprising at least one polymerizable ethylenicallyunsaturated monomer, said method comprising the steps of:

introducing a feed comprising at least one polymerizable ethylenicallyunsaturated monomer into a distillation apparatus;

introducing a polymerization inhibiting effective amount of at least oneinhibitor selected from the group consisting of C-nitrosoaniline andquinone imine oxime compounds into said distillation apparatus; and

distilling said feed under distillation conditions in the presence ofsaid inhibitor to recover from said distillation apparatus an overheadproduct of high purity ethylenically unsaturated monomer and a residualbottoms fraction having a reduced content of polymeric material. Inaccordance with a further embodiment, the residual bottoms fraction isrecycled back into said distillation apparatus to reuse unspentinhibitor.

In another aspect, the present invention is directed to a composition ofmatter comprising:

A) at least one inhibitor selected from the group consisting ofC-nitrosoaniline and quinone imine oxide compounds; and

B) at least one inhibitor selected from the group consisting of quinonealkides, nitroxyl compounds, nitroaromatic compounds, hydroxylaminecompounds, phenylenediamine compounds, quinone compounds, andhydroquinone compounds.

In another aspect, the present invention is directed to a composition ofmatter comprising:

A) an ethylenically unsaturated monomer and

B) an effective inhibiting amount, sufficient to prevent prematurepolymerization during distillation or purification of said ethylenicallyunsaturated monomer, of at least one inhibitor selected from the groupconsisting of C-nitrosoaniline and quinone imine oxime compounds usedtogether with an effective amount of oxygen or air to enhance theinhibiting activity of said inhibitor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The C-nitrosoaniline compounds can be prepared by C-nitrosation of thecorresponding anilines in any typical manner used for the C-nitrosationof aromatic amines. For example, reaction of the amine with cold nitrousacid produces an N-nitroso compound that rearranges to apara-nitrosoaniline under the influence of an excess of hydrochloricacid. In some cases, it is more convenient to effect the nitrosation andrearrangement in one step by conducting the reaction in methanolsolution in the presence of an excess of hydrogen chloride underanhydrous conditions. This procedure is described in U.S. Pat. No.2,046,356.

Those skilled in the art will be aware that nitrosoaniline derivativesare understood to tautomerize to quinone imine oxime derivatives, i.e.,

See, for example, Sidgwick, N. V., The Organic Chemistry of Nitrogen,Third Edition, Clarendon Press, Oxford, 1966. Thus, both forms can bepresent, especially in solution at elevated temperatures, and can beexpected to contribute to the inhibiting activity of these compounds.Furthermore, the quinone imine oxime tautomeric form can be enhanced byalkylation or acylation at the oxygen of the oxime. Thus, these quinoneimine oxime forms and their derivatives are embodied in the presentinvention.

The nitrosoaniline and quinone imine oxime inhibitors of the presentinvention can be used alone or in combination with at least one nitroxylcompound, at least one nitroaromatic compound, at least one quinonealkide, at least one quinone derivative, at least one hydroquinonederivative, at least one hydroxylamine compound, at least onephenylenediamine compound, air or oxygen, or a mixture of the foregoing.These inhibitors are suitable for use over a wide range of temperatures,but distillation temperatures employed with the ethylenicallyunsaturated monomers that are stabilized by the process of the presentinvention typically range from about 60° C. to about 180° C., preferablyfrom about 70° C. to about 165° C. and, more preferably, from about 80°C. to about 150° C. Such distillations are generally performed at anabsolute pressure in the range of about 10 to about 1,200 mm of Hg.

The nitrosoanilines employed in the practice of the present inventionare preferably of the structure:

wherein R₁ and R₂ are independently selected from the group consistingof hydrogen, alkyl, aryl, acyl, hydroxyl, alkoxy, nitroso, and sulfonyl,or R₁ and R₂ can form a cyclic ring that is aryl, cycloalkyl, polyaryl,or heterocyclic;

R₃ through R₇ are independently selected from the group consisting ofhydrogen, alkyl, aryl, acyl, hydroxyl, alkoxy, acyloxy, NR₈(R₉), nitro,nitroso, halogen, and sulfonyl, or any two adjacent R's can form acyclic ring that is aryl, cycloalkyl, polyaryl, or heterocyclic,provided that at least one of R₃ through R₇ must be a nitroso group; and

R₈ and R₉ are independently selected from the group consisting ofhydrogen, alkyl, aryl, acyl, and nitroso. Preferably R₈ is hydrogen andR₉ is alkyl.

The quinone imine oximes employed in the practice of the presentinvention are preferably of the structure:

wherein

R₁₂₀ R₁₂₁, R₁₂₂, and R₁₂₃ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, cycloalkyl, heterocyclic,substituted alkyl, substituted aryl, OR₁₁₀, NR110R₁₁₁, SR110, NO. NO₂,CN, COR₁₁₂, and halogen, or R₁₂₀ and R₁₂₁ can be taken together and/orR₁₂₂ and R₁₂₃ can be taken together to form one or two ring structures,respectively, either of which can be of five to seven members;

R₁₁₀ and R₁₁₁ are independently selected from the group consisting ofhydrogen, alkyl, aryl, acyl, benzyl, cyclic, heterocyclic, substitutedalkyl or aryl where the substituents are C, O, N, S, or P, and COR₁₀₂,or R₁₁₀ and R₁₁₁ can be taken together to form a ring structure of fiveto seven members;

R₁₁₂ is R₁₀₂, OR₁₀₂, or NR₁₀₂R₁₀₃; and

R₁₀₂ and R₁₀₃ are independently selected from the group consisting ofhydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₂ andR₁₀₃ can be taken together to form a ring structure of five to sevenmembers.

The nitroxyl compounds that can be employed in combination with thenitrosoanilines and quinone imine oximes in the practice of the presentinvention are preferably of the structure:

wherein R₁ and R₄ are independently selected from the group consistingof hydrogen, alkyl, and heteroatom-substituted alkyl and R₂ and R₃ are(1) independently selected from the group consisting of alkyl andheteroatom-substituted alkyl, or (2) taken together, form a ringstructure with the nitrogen; and X₁ and X₂ (1) are independentlyselected from the group consisting of halogen, phosphorus (in any of itsoxidation states), cyano, COOR₇, —S—COR₇, —OCOR₇, (wherein R₇ is alkylor aryl), amido, —S—C₆H₅, carbonyl, alkenyl, or alkyl of 1 to 15 carbonatoms, or (2) taken together, form a ring structure with the nitrogen.

In a particularly preferred embodiment, the nitroxyl compound has thestructural formula:

wherein R₁ and R₄ are independently selected from the group consistingof hydrogen, alkyl, and heteroatom-substituted alkyl and R₂ and R₃ areindependently selected from the group consisting of alkyl andheteroatom-substituted alkyl, and the

portion represents the atoms necessary to form a five-, six-, orseven-membered heterocyclic ring.

The quinone alkide compounds that can be employed in combination withthe nitrosoanilines and quinone imine oximes in the practice of thepresent invention are preferably of the structure:

wherein

X is oxygen,

Y is CR₁₂₄R₁₂₅,

R₁₂₀, R₁₂₁, R₁₂₂, and R₁₂₃ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, cycloalkyl, sulfonyl, heterocyclic,substituted alkyl, substituted aryl, OR110, NR₁₁₀R₁₁₁, SR₁₁₀, NO, NO₂,CN, COR₁₁₂, and halogen, or R₁₂₀ and R₁₂₁ can be taken together and/orR₁₂₂ and R₁₂₃ can be taken together to form one or two ring structures,respectively, either of which can be of five to seven members;

R₁₂₄ and R₁₂₅, are independently selected from the group consisting ofhydrogen, alkyl, aryl, cycloalkyl, heterocyclic, substituted alkyl,substituted aryl, OR₁₁₀, NR₁₁₀R₁₁₁, SR110, NO₂, NO, CN, COR₁₁₂, halogen,and/or can be taken together to form a ring structure of five to sevenmembers;

R₁₁₀ and R₁₁₁ are independently selected from the group consisting ofhydrogen, alkyl,aryl, acyl, benzyl, cyclic, heterocyclic, substitutedalkyl or aryl where the substituents are C, O, N, S, or P, and COR₁₀₂,or R110 and R₁₁₁ can be taken together to form a ring structure of fiveto seven members;

R₁₁₂ is R₁₀₂, OR₁₀₂, or NR₁₀₂R₁₀₃; and

R₁₀₂ and R₁₀₃ are independently selected from the group consisting ofhydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₂ andR₁₀₃ can be taken together to form a ring structure of five to sevenmembers.

The nitroaromatic compounds that can be employed in combination with thenitrosoanilines and quinone imine oximes in the practice of the presentinvention are preferably of the structure:

wherein R₃ through R₇ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, acyl, hydroxyl, alkoxy, acyloxy, NR₈(R₉), nitro, nitroso, halogen, and sulfonyl, or any two adjacent R'scan form a cyclic ring that is aryl, cycloalkyl, polyaryl, orheterocyclic; and

R₈ and R₉ are independently selected from the group consisting ofhydrogen, alkyl, aryl, acyl, and nitroso. Preferably R₈ is hydrogen andR₉ is alkyl. Preferably, R₃ is hydroxyl, R₆ is nitro, and R₄ is alkyl.

The hydroxylamine compounds that can be employed in combination with thenitrosoanilines and quinone imine oximes in the practice of the presentinvention are preferably of the structure:

wherein R₁₀₀ and R₁₀₁ are independently selected from the groupconsisting of hydrogen, alkyl, alkylidene, benzylidene, aryl, benzyl,COR₁₀₂, COOR₁₀₂, CONR₁₀₂R₁₀₃, cyclic, heterocyclic, hydroxyalkyl, andsubstituted alkyl or aryl where the substituents are C, O, N, S, or P,or

R₁₀₀ and R₁₀₁ can be taken together to form a ring structure of five toseven members.

The phenylenediamine compounds that can be employed in combination withthe nitrosoanilines and quinone imine oximes in the practice of thepresent invention are preferably of the structure:

wherein R₁ and R₂ are independently selected from the group consistingof hydrogen, alkyl, aryl, acyl, hydroxyl, alkoxy, nitroso, and sulfonyl,or R₁ and R₂ can form a cyclic ring that is aryl, cycloalkyl, polyaryl,or heterocyclic;

R₃ through R₇ are independently selected from the group consisting ofhydrogen, alkyl, aryl, acyl, hydroxyl, alkoxy, acyloxy, NR₈(R₉), nitro,nitroso, halogen, and sulfonyl, or any two adjacent R's can form acyclic ring that is aryl, cycloalkyl, polyaryl, or heterocyclic,provided that at least one of R₃ through R₇ must be an NR₈(R₉) group;and

R₈ and R₉ are independently selected from the group consisting ofhydrogen, alkyl, aryl, acyl, and nitroso. Preferably, R₁ is hydrogen, R₂is alkyl or aryl, R₈ is hydrogen, and R₉ is alkyl.

The quinone compounds that can be employed in combination with thenitrosoanilines and quinone imine oximes in the practice of the presentinvention are preferably of the structure:

wherein R₁₂₀, R₁₂₁, R₁₂₂, and R₁₂₃ are independently selected from thegroup consisting of hydrogen, alkyl, aryl, cycloalkyl, sulfonyl,heterocyclic, substituted alkyl, substituted aryl, OR₁₁₀, NR₁₁₀R₁₁₁,SR₁₁₀, NO, NO₂, CN, COR₁₁₂, and halogen, or R₁₂₀ and R₁₂₁ can be takentogether and/or R₁₂₂ and R₁₂₃ can be taken together to form one or tworing structures, respectively, either of which can be of five to sevenmembers;

R₁₁₀ and R₁₁₁ are independently selected from the group consisting ofhydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, substituted alkylor aryl where the substituents are C, O, N, S, or P, and COR₁₀₂, or R₁₁₀and R₁₁₁ can be taken together to form a ring structure of five to sevenmembers;

R₁₁₂ is R₁₀₂, OR₁₀₂, or NR₁₀₂R₁₀₃; and

R₁₀₂ and R₁₀₃ are independently selected from the group consisting ofhydrogen, alkyl, aryl, benzyl, cyclic, heterocyclic, and substitutedalkyl or aryl where the substituents are C, O, N, S, or P, or R₁₀₂ andR₁₀₃ can be taken together to form a ring structure of five to sevenmembers.

The hydroquinone compounds that can be employed in combination with thenitrosoanilines and quinone imine oximes in the practice of the presentinvention are preferably of the structure:

wherein R₃ through R₇ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, acyl, hydroxyl, alkoxy, acyloxy, NR₈(R₉), nitro, nitroso, halogen, and sulfonyl, or any two adjacent R'scan form a cyclic ring that is aryl, cycloalkyl, polyaryl, orheterocyclic, provided that at least one of R₃ through R₇ must be an OHgroup; and R₈ and R₉ are independently selected from the groupconsisting of hydrogen, alkyl, aryl, acyl, and nitroso. Preferably,either R₅ is OH and R₃ and R₆ are alkyl or R₃ is OH and R₅ is alkyl.

In the foregoing, alkyl (or substituted alkyl) groups, or the alkylmoieties of alkoxy groups, preferably contain one to 15 carbon atoms,e.g., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl,decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, and the like,and isomers thereof, e.g., t-butyl, 2-ethylhexyl, and the like. It ismore preferred that the alkyl (or substituted alkyl) groups be of one tofive carbon atoms (e.g., methyl, ethyl, propyl, butyl, pentyl, andisomers thereof). Substituents on the substituted alkyl groups can beany moiety that will not interfere with the functions of the compounds.Aryl groups are preferably of from six to 10 carbon atoms, e.g., phenylor naphthyl, which, in addition, may be substituted with noninterferingsubstituents, e.g., lower alkyl groups, halogens, and the like.

The effective amount of inhibitor(s) used in the practice of the presentinvention comprising the nitrosoaniline and/or quinone imine oximecompound(s), alone or in combination with a nitroxyl, and/ornitroaromatic, and/or quinone alkide, and/or quinone, and/orhydroquinone, and/or hydroxylamine, and/or phenylenediamine compound(s),is typically about 1 to 2,000 ppm, based on the weight of theethylenically unsaturated monomer, although amounts outside this rangemay be appropriate depending upon the conditions of use. The amount ispreferably in the range of from about 5 to about 1,000 ppm, based on theweight of the ethylenically unsaturated monomer.

The effective amount of air or oxygen used in the practice of thepresent invention is typically about 1 to 2,000 ppm, based on the weightof the ethylenically unsaturated monomer, although amounts outside thisrange may be appropriate depending upon the conditions of use. Theamount is preferably in the range of from about 1 to about 1,000 ppm,based on the weight of the ethylenically unsaturated monomer.

Preferred embodiments of the instant invention comprise a processwherein a mixture is used that is from 1 to 99 percent by weight of atleast one nitrosoaniline compound and 99 to 1 percent by weight of atleast one additional compound. A more preferred mixture comprises from 5to 75 percent by weight of at least one nitrosoaniline compound and 95to 25 percent by weight of at least one additional compound. A stillmore preferred mixture comprises from 5 to 50 percent by weight of atleast one nitrosoaniline compound and 95 to 50 percent by weight of atleast one additional compound.

The ethylenically unsaturated monomer, the premature polymerization ofwhich is an object of the present invention, can be any such monomer forwhich unintended polymerization during its manufacture, storage, and/ordistribution is a problem. Among those monomers that will benefit fromthe practice of the present invention are: styrene, α-methylstyrene,styrene sulfonic acid, vinyltoluene, divinylbenzenes, polyvinylbenzenes,alkylated styrene, 2-vinylpyridine, acrylonitrile, methacrylonitrile,methyl acrylate, ethyl acrylate, methyl methacrylate, ethylmethacrylate, acrylic acid, methacrylic acid, butadiene, chloroprene,isoprene, and the like.

The ethylenically unsaturated monomers will not necessarily bestabilized indefinitely by the presence of the inhibitor blend,especially when the monomers are heated as in distillation, but they canbe considered to be stabilized as long as there is a measurable increasein the time for which they can be heated before the onset ofpolymerization in a static system and/or the amount of polymer made atconstant temperature remains constant over time in a dynamic system.

Those skilled in the art will understand that, if desired, additionalfree radical scavengers can be included in the stabilized compositions.For example, air or O₂, as disclosed in U.S. Pat. Nos. 5,545,782 and5,545,786, can be added, as can the dihetero-substituted benzenecompounds having at least one transferable hydrogen, e.g., a quinonederivative such as the mono-methyl-ether of hydroquinone disclosed inEuropean Patent Application 0 765 856 A1, and other inhibitorswell-known to those skilled in the art. The disclosures of the foregoingare incorporated herein by reference in their entirety.

The polymerization inhibitor composition can be introduced into themonomer to be protected by any conventional method. It can be added as aconcentrated solution in suitable solvents just upstream from the pointof desired application by any suitable means. For example, theindividual inhibiting components can be injected separately or incombination to the monomer feed tank prior to injection into adistillation train. The individual inhibiting components can also beinjected separately into the distillation train along with the incomingfeed or through separate entry points, provided there is an efficientdistribution of the inhibitors. Since the inhibitors are graduallydepleted during the distillation operation, it is generally advantageousto maintain the appropriate amount of the inhibitor mixture in thedistillation apparatus by adding inhibitors during the course of thedistillation process. Adding inhibitors can be done either on agenerally continuous basis or intermittently, in order to maintain theconcentration of inhibitor mixture above the minimum required level.

The distillation method of the present invention is suitable for use invirtually any type of separation of a polymerizable ethylenicallyunsaturated monomer wherein the monomer is subjected to temperaturesabove room temperature. Thus, the process of the present invention hasbeen found particularly useful in vacuum distillation techniques, thepreferred method for separating unstable organic liquid mixtures. Theamount of polymerization inhibitor added may vary over a wide rangedepending upon the conditions of distillation. Generally, the degree ofstabilization is proportional to the amount of inhibitor added. Inaccordance with the present invention, it has been found that inhibitorconcentrations generally between about 50 ppm and about 3000 ppm byweight have generally provided suitable results, depending primarilyupon the temperature of the distillation mixture and the degree ofinhibition desired. More often, however, with the inhibitor of thepresent invention it is used in concentrations of 100 to 1000 ppm.

During vacuum distillation of ethylenically unsaturated monomer, thetemperature of the reboiler is preferably maintained from about 65° C.to about 130° C. by controlling reboiler pressure at from about 30 mm toabout 400 mm of Hg. Under such conditions, in a distillation apparatushaving a distillation zone containing from about 50 to about 100distillation stages, inhibitor mixture concentrations of from about 100ppm to about 2000 ppm by weight are suitable, whereas concentrations offrom about 100 ppm to about 600 ppm by weight are preferably, 200 to 600ppm by weight, in the case of styrene distillation and concentrations inthe range of from about 200 ppm to about 1000 ppm by weight arepreferred for distillation of divinylbenzene. The foregoing ranges arebased upon distillation temperatures of from about 65° C. to about 150°C. and residence times of between about 2 and 4 hours. Obviously, in thelower portions of the temperature and residence time ranges, smalleramounts of inhibitor may be utilized. Obviously, amounts of inhibitorgreater than those specified hereinabove may be employed, although theadvantages of adding the additional inhibitor are not significant andare outweighed by the corresponding increase in cost.

The polymerization inhibitor of the present invention may be introducedinto the distillation apparatus in any convenient manner which permitsefficient distribution of the inhibitor throughout the apparatus.Typically and most advantageously, the required amount of inhibitor issimply added to the reboiler area of the distillation column, althoughequivalent results may be obtained by incorporating the inhibitor intothe incoming hot stream of monomer. Also, the inhibitor may be added atboth reboiler and directly into the distillation column. Either and/orboth methods of addition provide a distribution of inhibitor which iscommensurate with the distribution of monomer within the distillationsystem and is essential for effective polymerization inhibition.

It is generally necessary to maintain the appropriate amount ofinhibitor in the distillation apparatus by adding inhibitor during thecourse of the distillation process, either on a generally continuousbasis or on an intermittent basis. The means by which the maintenance ofthe necessary concentration of the inhibitor system is carried out is ofno particular importance as long as the concentration of inhibitor iskept above or about the minimum required level.

One method by which the amount of inhibitor which is gradually depletedduring distillation and the increased cost necessitated thereby may beminimized is by recycling a portion of the distillation residue or tarback into the distillation system. It has been found that thedistillation residue contains a substantial quantity of polymerizationinhibitor which may be re-utilized in the distillation system with aconcomitant reduction in the process requirements for additionalinhibitor. Moreover, by recycling a portion of the tar, the amount ofinhibitor within the distillation system may be significantly increased,thereby enhancing protection against polymerization within the system.

The tar may be recycled back into the distillation system at anydesirable point therein such as would be obvious to those skilled in theart. However, in a typical distillation train comprising a firstfractionation column, a recycle column, and a finishing column, adequateinhibitor protection within the recycle column has been found to beessential to the elimination of thermal polymer, since the highdistillation temperatures necessary to achieve adequate fractionationbetween the similar boiling compounds separated therein causes theformation of a substantial portion of the total thermal polymer formedwithin the distillation system as a whole. Indeed, with conventionalprocesses, approximately 80% of the total thermal polymer formed isattributable to the recycle column. Accordingly, in the preferredembodiment, the portion of tar recycled is recycled to at least therecycle column, and preferably into the lower regions of the recyclecolumn in order to provide a locus of inhibitor distribution whichcorresponds to the distribution of ethylenically unsaturated monomertherein. Optionally, additional tar may be recycled for addition backinto the distillation system at other points, such as, for example, backinto the first fractionation column.

One convenient method by which the tar may be recycled back into thedistillation system is simply by incorporating the tar into an incomingfeed of monomer or inhibitor. The amount of tar which is recycled backinto the distillation system relative to the amount of feed may compriseany desirable amount. A larger amount of tar recycle will increase theloading of inhibitor within the distillation system. However, largeramounts of tar recycle will also increase the volume of bottomsmaterial, and the amount of tar recycle will necessarily be constrainedthereby.

The high purity overhead product withdrawn from the distillationapparatus will generally contain above about 97% and typically aboveabout 99% by weight ethylenically unsaturated monomer, depending uponthe ultimate use. The bottoms product may contain polymeric material,undistilled monomer and unspent inhibitor. This fraction is withdrawnfrom the distillation apparatus for further processing. In oneparticularly preferred embodiment of the present invention, a portion ofthe bottoms product, containing substantial amounts of re-useableinhibitor, is recycled for introduction into the distillation apparatus.The recycled portion of the bottoms product may be added to thedistillation apparatus by any method known to those skilled in the art.Best results are obtained by adding the recycled portion at a locationin the distillation apparatus which will yield a distribution ofinhibitor which coincides with the distribution of monomer therein. Byrecycling the inhibitor-containing bottoms, the inhibitor may thus bereused, accruing thereby a significant reduction in the processrequirements for inhibitor.

The advantages and the important features of the present invention willbe more apparent from the following examples.

EXAMPLES Procedure for Dynamic Reboiler Test with Feed Shut-Off

Preparation of Feed Solution.

T-Butylcatechol (TBC) is removed from commercially available styrene bydistillation under vacuum. Removal of TBC is verified by caustictitration. The desired amount of inhibitor(s) is added to this TBC-freestyrene either directly or by first making a concentrated solution ofthe inhibitor in TBC-free styrene followed by further dilution withTBC-free styrene.

Procedure for Reboiler Test under Ambient Conditions.

A quantity of the Feed Solution containing inhibitor (blend) at thedesired charge (stated as a wt/wt total inhibitor to styrene) is addedto a round-bottom flask (the “Pot”) and heated to the desiredtemperature (usually 116° C.) and brought to reflux by adjusting thepressure/vacuum. Once the Pot contents are at temperature, a continuousstream of fresh Feed Solution is begun at a rate that will add thevolume of the initial Pot solution to the Pot over a period of timecalled the residence time (typically one hour). At the same time thatthe fresh Feed Solution flow is begun, the Bottoms Stream flow is alsobegun. The Bottoms Stream is solution in the Pot that is removed at thesame rate as the fresh Feed Solution is added. The equal flows of Feedand Bottoms streams cause the quantity in the Pot to remain constantover the time of the experiment while allowing continuous replenishmentof inhibitor. This procedure simulates the way inhibitors are used in adistillation train of a plant producing vinyl monomers. The experimentcontinues with flow in and out of the Pot for a specified period oftime, typically seven hours. Samples are collected hourly from theBottoms Stream. These samples are analyzed for polymer content via themethanol turbidity method. The amount of polymer in the samples is anindication of effectiveness of the inhibitor being tested. The lower theamount of polymer in the hourly samples, the more effective theinhibiting system should be during normal operation of a continuousmanufacturing or purification process.

It should be noted that the methanol turbidity method for polymeranalysis usually involves absorbance readings at 420 nm. We have foundthat some C-nitrosoanilines have absorbances that interfere with polymeranalysis at this wavelength. Thus, in many instances, polymer wasquantified at 600 nm instead of 420 nm.

Procedure for Reboiler Test with Air Injection

This procedure is the same as that under ambient conditions except thata gas sparging tube is inserted into the contents of the Pot. Air isinjected through this sparging tube at a rate of 6 cc/min throughout thetest.

Procedure for Reboiler Test with Argon Injection

This procedure is the same as that under ambient conditions except thatargon gas is sparged through the Feed Solution and the apparatus at 6cc/min for 15 minutes prior to heating the Pot. Throughout the remainderof the test, argon is injected into the contents of the Pot via asparging tube at a rate of 6 cc/min while the Feed Solution ismaintained under an argon blanket.

Procedure for Feed Shut-Off

At the end of the Reboiler Test Run (typically seven hours), a sample iscollected from the Bottoms Stream. This sample corresponds to FeedShut-Off Time=0 minutes. The flows of fresh Feed Solution and BottomsStream are stopped. The vacuum and temperature are monitored andadjusted to maintain boiling at the desired temperature of theexperiment. If gas injection is being used, the injection of gas(es) iscontinued at the designated rate throughout feed shut-off. Samples areperiodically removed from the Pot (typically every five minutes). Thesesamples are analyzed for polymer content via the methanol turbiditymethod. A longer period of time before initiation of significant polymerformation is an indication of a more effective inhibiting system in theevent of a loss of feed in the plant. Additionally, the lower thepolymer number at a specific length of time after feed shut-off, themore effective the inhibitor system at providing protection for thatlength of time.

The results of experiments showing the improved inhibition provided bythe present invention are shown in Tables 1 through 7. In these tables,the following abbreviations apply:

NA-1 is N-phenyl-4-nitrosoaniline

NA-2 is N-(1,4-dimethylpentyl)-4-nitrosoaniline

DNBP is 2,4-dinitro-6-sec-butylphenol

QM is 4-benzylidene-2,6-di-tert-butylcyclobexa-2,5-dienone

nitroxyl is 4-oxo-TEMPO.

QIO is N-acetyloxy-N′-phenyl-1,4-diiminocyclohexa-2,5-diene.

DEHA is N,N-diethylhydroxylamine.

PDA is N-phenyl-N′-(1,4-dimethylpentyl)-para-phenylenediamine.

Naugard I-31 is a blend of PDA and DNBP.

Quinone is 2,5-di-tert-butyl-1,4-benzoquinone.

Hydroquinone is 2,5-di-tert-butyl-1,4-hydroquinone.

TABLE 1 Combinations of Nitrosoanilines with Quinonemethides Polymermeasurements taken at 420 nm; runs made under ambient conditions WeightPercent Polymer Example Inhibitor Dosage At steady 40 min. after NumberSystem (ppm) state feed shut-off 1 QM 250 1.09  1.52  2 NA-2 250 0.0334.35  3 NA-2/QM 100/150 0.002 0.51  4 QM 500 0.33  0.59  5 NA-1 5000.062 0.032 6 NA-1/QM 250/250 0.014 0.045

As can be seen from examples 2 and 5 of Table 1, the C-nitrosoanilines,NA-1 and NA-2, are very effective inhibitors at steady state. Examples 3and 6 of Table 1 show that the combination of nitrosoaniline andquinonemethide gives the preferred combination of highly effectivesteady state performance along with excellent protection in case of lossof feed, giving a combined performance which is better than theperformance obtained from either component by itself.

TABLE 2 Combinations of Nitrosoanilines with Nitroxyls Polymermeasurements taken at 420 nm; runs made under ambient conditions WeightPercent Polymer Example Inhibitor Dosage At steady 40 min. after NumberSystem (ppm) state feed shut-off  1 nitroxyl 150 0.005 1.8  2 nitroxyl100 0.002 2.7  3 nitroxyl  75 0.003 2.9  4 nitroxyl  50 0.002 3.9  5NA-1 250 0.014 1.54  6 NA-1/nitroxyl 200/50 0.012 0.53  7 QIO 250 0.53 2.5  8 QIO/nitroxyl 200/50 0.021 1.3  9 NA-2/DNBP  85/150 0.093 0.51 10nitroxyl/NA-2/  8/75/150 0.065 0.51 DNBP 11 NA-2/DNBP  75/200 0.049 0.3712 nitroxyl/NA-2/  10/50/200 0.043 0.22 DNBP 13 NA-2/DNBP 100/200 0.0170.36 14 nitroxyl/NA-2/  10/75/200 0.013 0.26 DNBP

Examples 1 to 4 of Table 2 indicate the effectiveness of nitroxyls asinhibitors. As stated previously, this quality of nitroxyls iswell-known. However, we have found that a dosage of less than 50 ppm inthis test provides insufficient protection, and the test becomesunstable and unsafe to run. Additionally, even at dosages of 100 ppm inthis test, the protection in feed shut-off is minimal. Thus, addition ofa small amount of nitroxyl (i.e., 50 ppm or less) in this test would notbe expected to provide significant enhancement to feed shut-offperformance. However, significant enhancement in feed shut-offperformance is obtained when 50 ppm of nitroxyl is added to aC-nitrosoaniline (compare examples 5 and 6) or to a quinoneimineoximederivative (compare examples 7 and 8). Furthermore, examples 9-14 showthat addition of 10 ppm or less of nitroxyl to blends ofC-nitrosoaniline and nitrophenol, where the total dose of the tri-blendis even less than the comparative dose of the C-nitrosoaniline andnitrophenol alone, provides better performance at steady state and equalor better performance in feed shut-off than obtained by the baselineblends of C-nitrosoaniline and nitrophenol alone.

TABLE 3 Combinations of Nitrosoanilines with Air Polymer measurementstaken at 420 nm Weight Percent Polymer Example Inhibitor Dosage Atsteady 40 min. after Number System (ppm) state feed shut-off 1 NA-2;argon injection 250 0.001 1.05 2 NA-2; argon injection 150 0.024 2.95 3NA-2; argon injection 100 0.04  3.4 4 NA-2; air injection 250 0.0020.008 5 NA-2; air injection 100 0.001 2.4 6 NA-2; air injection  500.002 1.9 7 NA-2; air injection  25 0.021 2.9

The examples in Table 3 show that C-nitrosoanilines are highly effectiveinhibitors under both aerobic (air injection) and anaerobic (argoninjection) conditions. However, the performance of theseC-nitrosoanilines in the presence of air is significantly improved inboth steady state and feed shut-off conditions (compare example 3 withexamples 5, 6, and 7). To our knowledge, this behavior ofC-nitrosoanilines has not been previously reported.

TABLE 4 Combinations of Nitrosoanilines with Nitroaromatics Polymeranalysis made at 420 nm, ambient conditions Weight Percent PolymerExample Inhibitor Dosage At steady 40 min. after Number System (ppm)state feed shut-off  1 DNBP 250 0.43  0.90  2 NA-2 250 0.033 4.35  3NA-2/DNBP 125/125 0.011 0.82  4 DNBP 500 0.21  0.40  5 NA-1 500 0.0620.03  6 NA-1/DNBP 250/250 0.008 0.11 Polymer analysis made at 600 nmWeight Percent Polymer Example Inhibitor Dosage At steady 50 min. afterNumber System (ppm) state feed shut-off  7 DNBP 250 0.39  0.80  8 NA-2;argon injection 250 0.001 2.06  9 NA-2; argon injection 100 0.04  3.9810 NA-2/DNBP; argon 100/150 0.007 0.65 injection 11 DNBP; air injection250 0.20  0.51 12 NA-2; air injection 250 0.002 0.48 13 NA-2; airinjection 100 0.001 3.53 14 NA-2; air injection  50 0.002 3.2  15NA-2/DNBP; air 100/150 0.001 0.24 injection 16 NA-2/DNBP; air  50/1500.001 0.24 injection

The examples in Table 4 show that the combination of a C-nitrosoanilineand a nitroaromatic compound provide enhanced performance over eithercomponent alone under ambient (dissolved air only), anaerobic (argoninjection), and aerobic (air injection) conditions.

Comparing Examples 1 through 3 and Examples 4 through 6 under ambientconditions, Examples 7 through 10 under anaerobic conditions, andExamples 11 through 16 under aerobic conditions, it is seen thatcombining a C-nitrosoaniline and a nitroaromatic compound gives thepreferred combination of highly effective steady state performance alongwith excellent protection in case of loss of feed, which is better thanthe combined performance obtained from either component alone when rununder the respective ambient, anaerobic, or aerobic conditions.

TABLE 5 Combinations of Nitrosoanilines with Hydroxylamines Polymermeasurements taken at 600 nm; runs made under conditions indicatedWeight Percent Polymer Example Inhibitor Dosage At steady 40 min. afterNumber System (ppm) state feed shut-off 1 DEHA; argon injection 250 1.33.00  2 NA-2; argon injection 250 0.001 2.06  3 NA-2/DEHA; argon 125/1250.003 1.45  injection 4 DEHA; air injection 250 <0.001 0.005 5 NA-2; airinjection 250 0.002 0.485 6 NA-2/DEHA; air 125/125 0.001 0.001 injection

The examples in Table 5 indicate that the blend of a C-nitrosoanilinewith a hydroxylamine provide equivalent or better performance thaneither component alone at the same total dosage in both steady state andfeed shut-off tests under both anaerobic (argon injection; Examples 1-3)and aerobic (air injection; Examples 4-6) conditions.

TABLE 6 Combinations of Nitrosoanilines with Phenylenediamines Polymermeasurements taken at 600 nm; runs made under air injection conditionsWeight Percent Polymer Example Inhibitor Dosage At steady 40 min. afterNumber System (ppm) state feed shut-off 1 Naugard I-31 500 0.024 0.16  2NA-2  50 0.002 1.9  3 NA-2/Naugard I-31  50/375 0.002 0.14  4 NA-2  250.021 2.9  5 NA-2/Naugard I-31  25/450 0.009 0.11  6 PDA 250 0.006 2.6 7 NA-2 250 0.002 0.008 8 NA-2/PDA 125/125 0.001 0.006

Examples 6-8 of Table 6 show that combination of a C-nitrosoaniline anda phenylenediamine provide better performance than either componentalone at the same total dosage in both steady state and feed shut-offtests. Examples 1-5 of Table 6 show that addition of just a small amountof C-nitrosoaniline to a blend of nitroaromatic and phenylenediamineprovides a tri-blend with the preferred combination of highly effectivesteady state performance along with excellent protection in case of lossof feed which the nitroaromatic/phenylenediamine blend could not providealone, even at a higher total dosage.

TABLE 7 Combinations of Nitrosoanilines with Quinones and HydroquinonesPolymer analysis made at 420 nm, ambient conditions Weight PercentPolymer Example Inhibitor Dosage At steady 40 min. after Number System(ppm) state feed shut-off 1 Quinone 300 3.98  4.30 2 NA-2 250 0.033 4.353 NA-2/Quinone  75/200 0.02  1.5  Polymer analysis made at 600 nm WeightPercent Polymer Example Inhibitor Dosage At steady 40 min. after NumberSystem (ppm) state feed shut-off 4 Hydroquinone 300 2.77  6.60 5 NA-2;argon injection 100 0.040 3.40 6 NA-2/Hydroquinone;  75/175 0.083 2.49argon injection 7 NA-2; air injection 100 0.001 2.4  8NA-2/Hydroquinone;  75/175 0.002 0.41 air injection

In view of the many changes and modifications that can be made withoutdeparting from principles underlying the invention, reference should bemade to the appended claims for an understanding of the scope of theprotection to be afforded the invention.

What is claimed is:
 1. A method for inhibiting the prematurepolymerization of ethylenically unsaturated monomers comprising addingto said monomers an effective amount of at least one inhibitor that isan quinone imine oxime compound.
 2. The method of claim 1 wherein thequinone imine oxime compound isN-acetyloxy-N′-phenyl-cyclohexa-2,5-diene-1,4-diimine.
 3. A method forinhibiting the premature polymerization of ethylenically unsaturatedmonomers comprising adding to said monomers an effective amount of atleast one inhibitor that is an quinone imine oxime compound and whereinthe inhibitor further comprises at least one additional compoundselected from the group consisting of quinone alkides, nitroxylcompounds, nitroaromatic compounds, hydroxylamine compounds,phenylenediamine compounds, quinone compounds, and hydroquinonecompounds.
 4. The method of claim 3 wherein the quinone imine oximecompound is N-acetyloxy-N′-phenyl-cyclohexa-2,5-diene-1,4-diimine. 5.The method of claim 3 carried out in the presence of oxygen.
 6. Themethod of claim 5 wherein the quinone imine oxime compound isN-acetyloxy-N′-phenyl-cyclohexa-2,5-diene-1,4-diimine.